System for fourier domain optical coherence tomography

ABSTRACT

Optical coherence tomography (OCT) is an imaging method which can image with micrometer-scale resolution up to a few millimeters deep into, for example, living biological tissues and preserved tissue samples. An improved apparatus and image reconstruction algorithm for parallel Fourier Domain OCT which greatly eases requirements for interferometer stability and also allows for more efficient parallel image acquisition is provided. The apparatuses and algorithms reconstruct images from interfered, low-coherence, multiwave length signals having a π radian phase difference relative to one another. Other numbers of signals and other phase differences may be alternatively used, with some combinations resulting in higher resolution and image stability. The apparatus also eliminates a need for bulk optics to modulate a phase delay in a reference arm of the optical path. Images may be reconstructed using two spectrometers, where each is coupled to a detector array such as a photodiode array.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional PatentApplication Ser. No. 60/473,457 filed May 28, 2003, which isincorporated herein by reference.

GOVERNMENT RIGHTS

[0002] This invention was made with United States Government supportunder Federal Grant No. BES 0134707 awarded by the National ScienceFoundation. The government has certain rights to this invention.

FIELD OF THE INVENTION

[0003] The invention relates to imaging systems and more particularly totomographic and interferometric imaging systems with high resolution.

BACKGROUND OF THE RELATED ART

[0004] Optical coherence tomography (OCT) is a method for noncontactoptical imaging taking advantage of sequential or scanned distancemeasurements developed primarily in the 1990's. In biological andbiomedical imaging applications, OCT allows for micrometer-scale imagingnoninvasively in transparent and translucent biological tissues. Thelongitudinal ranging capability of OCT is based on low-coherenceinterferometry, in which light from a broadband source is split betweenilluminating the sample of interest and a reference path. Theinterference pattern of light reflected or backscattered from the sampleand light from the reference delay contains information about thelocation and scattering amplitude of the scatterers in the sample. Inconventional (time-domain) OCT, this information is extracted byscanning the reference path delay and detecting the resultinginterferogram pattern as a function of that delay.

[0005] The envelope of the interferogram pattern thus detectedrepresents a map of the reflectivity of the sample versus depth, calledan “A-scan”, with depth resolution given by the coherence length of thesource. In conventional OCT systems, multiple A-scans are acquired whilethe sample beam is scanned laterally across the tissue surface, making acontinuous series of distance measurements and building up atwo-dimensional map of reflectivity versus depth and lateral extentcalled a “B-scan.” The lateral resolution of the B-scan is given by theconfocal resolving power of the sample arm optical system, which isusually given by the size of the focused optical spot in the tissue.

[0006] Time-domain OCT systems have been designed to operate at moderate(˜1 image/sec) and high speeds (up to video rate), and have been appliedfor imaging in biological applications such as imaging of embryonicdevelopment, as well as in medical diagnostic applications such asimaging the structures of the anterior and posterior segments of theeye, the skin, the gastrointestinal tract, and other tissues.Specialized probes, endoscopes, catheters, and biomicroscope attachmentshave been designed to allow for OCT imaging in these applications.

[0007] The time-domain approach in conventional OCT has been by far themost successful to date in supporting biological and medicalapplications, and all in-vivo human clinical trials of OCT to date haveutilized this approach. However, the time-domain approach in OCT suffersfrom some limitations. First, the requirement for mechanical scanning,such as with bulk optics, of the reference delay in conventional OCTintroduces complexity, expense, and reduced reliability, especiallythose which image at high speed and acquire A-scans at kilohertz rates.The mechanical scanning reference delay line is typically the mostcomplex optical apparatus in high-speed conventional OCT systems, andcan be quite bulky as well. Second, since conventional OCT images arebuilt up serially using a single detector and collecting one pixel ofimage information at a time, no advantage is taken of modern 1D and 2Darray detection technologies which dominate other forms of opticalimaging.

[0008] The serial collection or scanning approach of time-domain OCT isalso very wasteful of sample arm light, in that an entire column ofpixels is illuminated by that light while reflected light is onlycollected from one pixel at a time. This wastefulness of sample armlight is costly because sources of broadband light suitable for use inOCT systems are typically expensive and limited in their output powercapability, and also because optical damage to tissue structures oftenlimits the maximum power which may be used in OCT imaging, particularlyin the retina. Where there is a limit on the amount of light which maybe used to illuminate the sample, the wastefulness of sample arm lighttranslates directly into increased image acquisition time. Further, theserial scanning approach in conventional OCT requires that the sampleunder investigation remains stationary during the acquisition of eachA-scan, otherwise motion artifacts may appear in the image. Finally,primarily because of the requirement for a mechanical delay scan,conventional high-speed OCT systems are typically expensive, bulky, andrequire frequent optical alignment.

[0009] A potential solution to this need for a new approach has beenvariously termed spectral radar, Fourier-domain OCT (FDOCT), complexFourier OCT, Optical Frequency-domain imaging, and swept-source OCT. InFDOCT, a different form of low-coherence interferometry is used in whichthe reference delay is fixed (except for potential wavelength-scaledelay modulation in some implementations), and information about thelocation and amplitude of scatterers in the sample is derived from theoptical spectrum of the light returning from the sample and mixing withthe reference. This spectral information is typically acquired byspectrally dispersing the detector arm light using a spectrometer anddetecting it with an array detector such as a charge-coupled device(CCD), or else by using a single detector and sweeping the sourcefrequency as a function of time

[0010] The A-scan data collected using FDOCT can be shown to be related(see below) to the inverse Fourier transform of the spectral data thusacquired. Initial implementations of FDOCT suffered from image artifactsresulting from: 1) large direct-current (DC) signals appearing on thedetector array arising from non-interfering light returning from thereference delay and the sample, thus dwarfing the much smallerinterferometric signals; and 2) autocorrelation of light signals betweendifferent reflections within the sample. As a result, initial results ofFDOCT imaging were filled with artifacts and were not comparable toimages obtained with time-domain OCT.

[0011] Recently, newer implementations of FDOCT have appeared which takeadvantage of techniques well known from phase-shifting interferometry(PSI) to eliminate the sources of both of the artifacts mentioned above.Since both artifacts resulted from light appearing on the detector arraywhich does not arise from interference between sample and reference armlight, the recently introduced technique of complex FDOCT eliminatesthese artifacts by acquiring multiple spectra with different phaseshifts introduced into the reference delay path.

[0012] In a simple implementation of FDOCT, the reference delay consistsof a mirror mounted on a piezoelectric actuator (PZT). One spectrum isacquired at a given position of the mirror, and then another is acquiredwith a path-length delay of π/2 (resulting in a round-trip phase shiftof π) introduced into the reference arm by the PZT. It isstraightforward to show that this π phase shift reverses the sign of theinterferometric light components but has no effect on the DC componentsof the detector arm light, so subtracting the spectra obtained at 0 andπ phase shifts results in a spectrum free of DC artifacts. This spectrumcan be considered the real part of the complex Fourier transform of theA-scan. Thus, taking the inverse Fourier transform reconstructs theoriginal A-scan. However, since only the real part of the complexFourier spectrum is acquired, the A-scan data reconstructed isrestricted to be symmetric. Specifically, f(−z)=f*(z), and thus onlyA-scan data for positive displacements (i.e., z>0) can be reconstructed.

[0013] As a further refinement of this phase-shifting technique, anadditional spectrum may be acquired for a path-length delay of π/4(corresponding to a round-trip phase shift of π/2). This spectrum (alsooptionally corrected for DC components by division by one of the otherspectra or by subtraction with a spectrum acquired with a path-lengthdelay of 3π/4) may be considered the imaginary part of the complexFourier transform of the A-scan. Thus, taking the inverse Fouriertransform of the complete complex spectrum (resulting from all two,three, or four phase measurements) allows for unambiguous reconstructionof all depths in the sample limited only by spatial samplingconsiderations. Additional refinements to this approach may be appliedwhich are commonplace in phase-shifting interferometry, such as the useof additional phase delays for increased accuracy in measuring thecomplex spectrum.

[0014] Complex FDOCT thus addresses several of the needs for OCT systemswith decreased complexity and cost and increased reliability, having amostly fixed reference delay and utilizing an array detector. However,serious limitations to these prior art complex FDOCT implementationsinclude: 1) a means is still required for displacing the reference delayby distances on the scale of a wavelength; all prior systems performthis function by using bulk optical devices outside of the reference armoptical fiber; and 2) the spectra obtained at different reference phasesare obtained sequentially, thus the sample and reference arms must bemaintained interferometrically motionless during the entire A-scanspectrum acquisition.

SUMMARY OF THE INVENTION

[0015] An object of the invention is to solve at least the aboveproblems and/or disadvantages and to provide at least the advantagesdescribed hereinafter.

[0016] Another object of the invention is to provide an approach to OCTwhich eliminates the need for a mechanically scanned reference delay andmakes use of array detection technologies to acquire signals from allilluminated axial pixels of an A-scan simultaneously.

[0017] Another object of the invention is to enable the construction ofOCT systems which are inexpensive, compact, and are mechanically stablesuch that they rarely require optical realignment.

[0018] Another object of the invention is to provide an improvement toFDOCT which does not require any means for modulation of the referencearm path length, or may accomplish such modulation within the existingreference arm optical fiber.

[0019] Another object of the invention is to provide an FDOCT systemwhich obtains the multiple phase delays required for elimination ofimage artifacts and/or removal of constraints on A-scan asymmetrysimultaneously, thus relaxing constraints on sample and reference motionduring A-scan acquisition.

[0020] To achieve the aforementioned objects, an improved system forFDOCT is provided which implements readout of multiple reference phasesin two or more detector channels simultaneously.

[0021] To further achieve the aforementioned objects, an improved systemfor FDOCT is provided which eliminates the need for a mechanicallyscanned reference delay and makes use of array detection technologies orwavenumber swept sources to acquire signals from all illuminated axialpixels of an A-scan simultaneously.

[0022] To further achieve the aforementioned objects, a method isprovided which takes advantage of inherent π phase differences betweendifferent ports of interferometers, and which also utilizes orthogonalpolarization channels within the reference delay to encode arbitraryphase delays is provided.

[0023] To further achieve the aforementioned objects, a system isprovided that utilizes photodiode arrays for optimal S/N ratio in FDOCT.

[0024] To further achieve the aforementioned objects, a system isprovided that utilizes silicon-based photodiode arrays for FDOCT in the830 nm OCT window and InGaAs arrays for FDOCT in the 1310 nm and 1550 nmspectral regions. Further advantages of the use of dual-stripe andtwo-dimensional CCD and photodiode arrays are also disclosed.

[0025] Additional advantages, objects, and features of the inventionwill be set forth in part in the description which follows and in partwill become apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

[0027]FIG. 1A is a schematic illustration of a first embodiment of aFDOCT system, in accordance with the present invention;

[0028]FIG. 1B is a schematic illustration of a second embodiment of aFDOCT system, similar to the embodiment of FIG. 1, that utilizes asupport frequency source, in accordance with the present invention;

[0029]FIG. 2 is a schematic illustration of a third embodiment of aFDOCT, system in accordance with the present invention;

[0030]FIG. 3 is a schematic illustration of a fourth embodiment of aFDOCT system, in accordance with the present invention;

[0031]FIG. 4 is a schematic illustration of a fifth embodiment of aFDOCT system, in accordance with the present invention;

[0032]FIG. 5 is a schematic illustration of a sixth embodiment of aFDOCT system, in accordance with the present invention;

[0033]FIG. 6 is a schematic illustration of a seventh embodiment of aFDOCT system, in accordance with the present invention;

[0034]FIG. 7 is a generalized schematic illustration of a eighthembodiment of a FDOCT system, in accordance with the present invention;and

[0035]FIG. 8 is a schematic illustration of an imaging spectrometer, inaccordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] Referring to FIG. 1, a first FDOCT 10 in accordance with oneembodiment of the present invention, is shown. The FDOCT 10 of FIG. 1includes an optical circulator 34 with three ports 34 a, 34 b and 34 c,and a first fiber coupler 36 having four Michelson interferometer ports37 a, 37 b, 37 c and 37 d. The optical circulator 34 and the first fibercoupler 36 together make up an optical manipulator 1. The first fibercoupler 36 is coupled to a first polarization controller 42 by a singlemode (SM) optical fiber 41. The first polarization controller 42 iscoupled to a first phase modulator 44 by a SM optical fiber 48. Thefirst phase modulator 44 is coupled to a SM optical fiber 51 whichterminates in a reflector 46. Together the first polarization controller42, first phase modulator 44 and reflector 46, plus SM optical fibers 48and 46, form a reference arm.

[0037] A second polarization controller 54 is coupled by a SM opticalfiber 52 to the first fiber coupler 36. The second polarizationcontroller 54 is optically coupled to a first lens 56 by a SM opticalfiber 65. The first lens 56 is configured to capture a signal exitingthe SM optical fiber 65, and direct the signal to scanning optics 58,preferably a moveable mirror. The scanning optics 58, together withsecond lens 61, directs the signal onto a sample 62 and receive areflected signal therefrom. The second polarization controller 54, firstlens 56, scanning optics 58, and second lens 60, a SM optical fiber 52and SM optical fiber 65 form a sample arm. Applicant notes that theterms “signal,” “beam,” and “light” are used synonymously to include allforms of electromagnetic radiation suitable for use in imaging systems.

[0038] Also connected by a SM optical fiber 64 to the first fibercoupler 36 is a first detector 66 having a first spectrometer 66 a andfirst array detector 66 b. A second detector 71 is coupled to theoptical circulator 34 by a SM optical fiber 68. The optical circulator34 may additionally be configured to receive a signal from a source 72through a SM optical fiber 74. The first and second detectors, 66 and71, and SM optical fibers 64 and 68 together form a detector portion ofthe FDOCT 10.

[0039] The FDOCT system 10 is preferably implemented using a SM fiberMichelson interferometer illuminated by a broadband short-coherencelength light source 72. Light from the source 72 may be evenly splitbetween sample and reference arms by the first fiber coupler 36. Thesample arm can optionally include a second polarization controller 54for controlling the polarization state of the optical signal andscanning mirror 58 and lens 61 for scanning and focusing the sample armsignal onto the sample 62.

[0040] The reference arm may have a fixed path length, preferablyobtained by placing a reflector 46 on the tip of the reference arm fiber51 (thus eliminating bulk optics entirely in the reference arm and thesubstantial losses incurred in coupling out of and back into the SMfiber 51).

[0041] The reference arm may also optionally include a firstpolarization controller 42 for matching the polarization state in thereference arm to that in the sample arm, and may also optionally includea first phase modulator 44 which is capable of selectively causingwavelength-scale variations in the reference delay under user control.The first phase modulator 44 can be placed in the sample arm in this andall subsequent implementations without any loss of functionality. Lightfrom the third port 34 c of the first circulator 34 and light from thethird Michelson interferometer port 37 c of the fiber coupler 36, having180° phase difference between them may be coupled into a pair ofdetectors, 71 and 66 respectively. Detectors 66 and 71 each preferablyinclude spectrometers 66 a and 71 a, and array detector, 66 b and 71 b,respectively. This configuration is designed to place copies of thephase-shifted optical spectrum onto a matched pair of array detectors.

[0042] The FDOCT 10 of FIG. 1 acquires a pair of FDOCT spectra, with πradians phase difference between them, on a pair of array detectors toeliminate most motion artifacts associated with conventional phase-shiftinterferometry. Spectrometers 66 a and 71 a, and array detectors 66 band 71 b are preferably matched as closely as possible in their opticaland electronic characteristics. This can be accomplished by usingspectrometers and detector arrays of matching design.

[0043] In operation, the FDCOT 10 acquires spectra having a relativephase delay of 180° between them from interferometer ports 37 c and 3 ofthe interferometer, and differences the spectra in order to eliminatesample and reference arm DC and sample arm autocorrelation terms. Theresulting difference spectrum is inverse Fourier transformed to acquirea one-sided A-scan. Care must be taken to assure that the length of thereference arm is adjusted so that no reflections are observed for z<0.

[0044] In an alternative mode of operation, the FDOCT 10 acquires a 180°relative phase delay between them and differences them in order toeliminate most sample and reference arm DC and sample armautocorrelation terms, as designated above. The first phase modulator 44in the reference arm is then adjusted for 90° of additional referencedelay. Simultaneous spectra having 90° and 270° phase delay between themare then acquired and differenced in order to eliminate most sample andreference arm DC and sample arm autocorrelation terms. The first andsecond difference spectra may then be taken as the real and imaginaryparts, respectively, of the complex Fourier transform of the two-sidedA-scan. An inverse Fourier transform may be performed on the complexdata to obtain the A-scan free of symmetry considerations. Additionalphase delays of the first phase modulator 44 may also be selected, andorthogonal pairs of spectra obtained, according to establishedalgorithms for phase-shift interferometry.

[0045]FIG. 1B shows a second FDOCT 15, in accordance with a secondembodiment of the present invention. The FDOCT 15 fifteen of FIG. 1B issimilar to the FDOCT system 10 of FIG. 1A, except that a swept-frequencysource 720 is used in place of the broadband short-coherence lengthlight source 72 of FIG. 1A. In addition, single-channel detectors 800Aand 800B are used in place of the spectrometer/array detectorcombinations of FIG. 1A.

[0046] The swept-frequency source 720 is preferably a narrowband lightsource whose frequency can be swept as a function of time. In theembodiment of FIG. 1B, the spectrum of the interferometer output(s) isobtained by monitoring the output of the detectors 800A, 800B as afunction of time while the frequency of the swept-frequency source 720is swept.

[0047] The additional embodiments discussed below will be shown with abroadband light source, and with spectrometer/array detector(s) thataree used to resolve the spectrum of the interferometer output. However,it should be appreciated that all of the embodiments described below canalso be implemented in a swept-source configuration, such as theconfiguration shown in FIG. 1B, by replacing the broadband source with aswept-frequency narrowband source, and by replacing each detector with asingle-channel time-resolved detector. When Fourier Domain OCT isperformed using a swept-source implementation, then all of the sameadvantages conferred by obtaining multiple simultaneous phasedifferences, either from multiple output ports of the variousinterferometer topologies, from polarization encoding of phase in theinterferometer arms, or a combination of both approaches, will apply.

[0048]FIG. 2 shows a second FDOCT embodiment 20, in accordance with thepresent invention. Similar to the FDOCT embodiment 10 shown in FIG. 1,the FDOCT embodiment 20 of FIG. 2 has a fiber coupler 36 connected to afirst polarization controller 42 by a SM optical fiber 41. The firstpolarization coupler 42 is connected to a phase modulator 44 by a SMoptical fiber 48. The phase modulator 44 has a fiber 51 extendingtherefrom terminating in a reflector 46. Additionally, the first fibercoupler 36 is connected to a second polarization controller 54 by a SMoptical fiber 52. The second polarization controller 54 is opticallycoupled to a first lens 56 by a SM optical fiber 64. The first lens 56directs an optical output signal from fiber 64 to scanning optics 58.Scanning optics 58 and lens 61 direct the optical signal to sample 62.Reflected optical signals from the sample 62 are coupled back into fiber64 via lens 61 scanning optics 58 and lens 56.

[0049] The first fiber coupler 36 is also connected to a first detector66 by a SM optical fiber 64. In a variation from the first FDOCTembodiment 10, the first fiber coupler 36 is connected to a second fibercoupler 76 through a SM optical fiber 38. The second fiber coupler 76 isconnected to a second detector 71 through a SM optical fiber 68, andincludes a 0° port 75 and a 180° port 77. The second fiber coupler 76also includes a SM optical fiber stub 78 connected to the 180° port 77.A low coherence source 72 may also be connected to the second fibercoupler 76 through a SM optical fiber 74.

[0050] The FDOCT embodiment 20 of FIG. 2 is similar in many respects tothe FDOCT embodiment 10 of FIG. 1, except that a second fiber coupler 76is used in the source arm to provide one of the orthogonal phasecomponents, in place of the first circulator 34 of FIG. 1. Use of asecond fiber coupler 76 may be preferable for decreasing system cost orif circulators are not available to meet the specified wavelength orbandwidth requirements. The penalty for use of the second fiber coupler76 in place of a circulator will result in a higher insertion loss (3 dBfor a fiber coupler versus −0.7 dB for a circulator) in the forwarddirection, plus a 3 dB loss in the reverse direction of the 0° port 75,which will need to be matched by an equal amount of attenuation of the180° port 64 in order to match DC levels on the detectors 66 and 71.

[0051] Thus, the FDOCT embodiment 20 may experience a total loss ofsource light of approximately 6 dB loss (not counting circulatorinsertion losses) as compared to the FDOCT embodiment 10. As in theFDOCT embodiment 10 of FIG. 1, the separate spectrometers 66 a and 71 a,and array detectors 66 b and 71 b of the first and second detectors 66and 71 could be replaced by an imaging spectrometer and a dual-row orthree-color detector array. Also, any of the three modes of operationdiscussed in connection with the FDOCT embodiment 10 of FIG. 1 may alsobe used in the FDOCT of FIG. 2.

[0052]FIG. 3 illustrates a third FDOCT embodiment 30, in accordance withthe present invention. Similar to the FDOCT embodiments 10 and 20discussed above, the FDOCT embodiment 30 includes a fiber coupler 36coupled to a second polarization controller 54 through a SM opticalfiber 52. The first fiber coupler 36 includes interferometer ports 37 a,37 b, 37 c and 37 d. The second polarization controller 54 is opticallycoupled to a first lens 56 through a SM optical fiber 56. The first lens56 directs and receives signals to and from a sample 62 through scanningoptics 58 and second lens 61.

[0053] Also attached to the first fiber coupler 36 is a reference armincluding a second phase modulator 84 connected to the first fibercoupler 36 with a SM optical fiber 82. The second phase modulator 84 isconnected to a third polarization controller 88 with a SM optical fiber86. The third polarization controller 88 is connected to a third fibercoupler 94 through a SM optical fiber 92. The third fiber coupler 94 isalso connected to the first fiber coupler 36 with a SM optical fiber 96.The third fiber coupler 94 is connected to a first detector 66 and asecond detector 71 through SM optical fibers 64 and 68, respectively.The FDOCT embodiment 20 of FIG. 2 may also include a source 72 coupledto the first fiber coupler 36 through a SM optical fiber 75.

[0054]FIG. 3 illustrates a FDOCT embodiment 30, in accordance with thepresent invention, which takes advantage of the intrinsic phasedifference between interferometer ports 37 a and 37 b of the first fibercoupler 36, but has a transmissive reference delay rather than areflective one. Other aspects of the FDOCT embodiment 30 are similar tothe previously discussed FDOCT embodiments 10 and 20. The FDOCTembodiment 30 of FIG. 3 may experience a loss of 3 dB of the sample armreflected light (which is returned into the source). However, this 3 dBloss is less than the corresponding configurations in other FDOCTembodiments, and is approximately the same loss experienced bytime-domain OCT in a conventional Michelson interferometer. Thus, theFDOCT embodiment 30 of FIG. 3 is the preferred implementation when acirculator is unavailable or undesirable. As in the previously discussedFDOCT embodiments 10 and 20, optimally the first and secondspectrometers 66 a and 71 a and first and second array detectors 66 band 71 b could be replaced by an imaging spectrometer and a dual-row orthree-color detector array. Also, any of the three modes of operationdiscussed above in connection with FDOCT embodiments 10 and 20, may alsobe used.

[0055]FIG. 4 illustrates a fourth FDOCT embodiment 40, in accordancewith the present invention. The FDOCT embodiment 40 includes a firstfiber coupler 36 which is coupled to a first polarization controller 42by a SM optical fiber 41. The first fiber coupler 36 has fourinterferometer ports 37 a, 37 b, 37 c, and 37 d. One interferometer port37 a is coupled to a first polarization controller 42 by a SM fiber 41,and the second interferometer port 37 b is coupled to a secondpolarization controller 54 by a SM fiber 52. The first polarizationcontroller 42 is coupled to a first phase modulator 44 by a SM opticalfiber 48. The second polarization controller 54 is coupled to the firstfiber coupler 36 by a SM optical fiber 52, and to a second circulator102 by a SM optical fiber 64. The second circulator 102 has a SM opticalfiber 110 optically coupled to a first lens 56, which directs andreceives a signal to and from a sample 62 through scanning optics 58 andsecond lens 61. The second circulator 102 is also coupled to a fourthfiber coupler 98 through a SM optical fiber 104. The first phasemodulator 44 is also coupled to the fourth fiber coupler 98 by a SMfiber 50.

[0056] The fourth fiber coupler 98 is coupled to first and seconddetector 66 and 71 through SM optical fibers 64 and 68, respectively.The FDOCT embodiment 40 may also include a first source 72 coupled tothe first fiber coupler 36 through a SM optical fiber 74 and a secondsource 106 coupled to the fiber coupler 36 through a SM optical fiber108.

[0057] The FDOCT embodiment 40 of FIG. 4 takes advantage of theintrinsic phase difference between interferometer ports 37 a and 37 b,and also has a transmissive delay. However, the FDOCT embodiment 40 usesa second circulator 102 to direct light onto the sample 62. The FDOCTembodiment 40 also places the second circulator 102 within one of thearms of the interferometer, where chromatic and polarization modedispersion effects within the second circulator 102 may be problematic.

[0058] However, the embodiment 40 makes highly efficient use of sourcelight (except for insertion losses in the circulator 102 itself), andalso allows for the introduction of a second source 106. This may bepreferable in order to increase the power of low-coherence light on thesample 62 from available light sources 72 and 106, and also may be usedto increase the bandwidth of illumination by using sources withdisplaced center wavelengths. As in the previously discussed FDOCTembodiments 10, 20 and 30, the separate spectrometers 66 a and 71 a andarray detectors 66 b and 71 b could be replaced by an imagingspectrometer and a dual-row or three-color detector array. Also, any ofthe three modes of operation discussed above in connection with thepreviously discussed FDOCT embodiments 10, 20 and 30 may be used in theFDOCT embodiment 40 of FIG. 4.

[0059] All the FDOCT embodiments discussed above may take advantage ofthe intrinsic π phase delay which is found between output ports ofMichelson and Mach-Zehnder interferometers. As described above, thisphase delay may be used to simultaneously obtain pairs of spectra whichmay be differenced to remove non-interferometric noise from the spectraldata. However, it also may be desirable to obtain pairs of spectra withπ/2 phase delay simultaneously, to allow for both removal ofnon-interferometric noise and also for unambiguous calculation of samplereflectivity without symmetry artifacts. The embodiments described belowtake advantage of polarization to encode arbitrary phase delays intospectra which may be measured simultaneously.

[0060]FIG. 5 illustrates a fifth FDOCT embodiment 50, in accordance withthe present invention. The FDOCT embodiment 50 includes a non-polarizingbeam splitter 114 optically coupled to a λ/n waveplate 116 and a fixedreference mirror 118. The non-polarizing beam splitter 114 is alsooptically coupled to a sample 62 through scanning optics 58 and secondlens 61. The non-polarizing beam splitter 114 is additionally opticallycoupled to a polarizing beam splitter 120. Polarizing beam splitter 120is optically coupled to a first detector 66 through lens 122, and asecond detector 121 through lens 124. The FDOCT embodiment 50 may alsoinclude a source 72 optically coupled to the non-polarizing beamsplitter 114 through a polarizer 112.

[0061] The FDOCT embodiment 50 is similar to a bulk-optic Michelsoninterferometer which may encode a 90° phase shift into two polarizationchannels, which are separated outside of the interferometer by thepolarizing beam splitter 120. The two polarization channels may bedirected into a matched pair of spectrometers 66 a and 71 a and arraydetectors, 66 b and 71 b. Light emitted from the preferablylow-coherence source 72 may be linearly polarized at 45° from thevertical by a polarizer 112 placed in the source arm. The non-polarizingbeamsplitter 114 splits this light evenly between sample and referencearms. A λ/n waveplate 116 (for n=2, 4, 8, etc.) may be placed in thereference arm, with its fast axis oriented vertically (i.e., at 0° tothe vertical).

[0062] Thus, the horizontal component of the light in the reference armmay experience a phase delay of 4π/n radians with respect to thevertical component after double-passing the λ/n waveplate 116. These twocomponents may be separated by the polarizing beamsplitter 120 in thedetector arm, which sends the phase-delayed components of the referencearm light, along with an equal division of the light reflected from thesample 62, into a matched pair of spectrometers 66 a and 71 a, and arraydetectors, 66 b and 71 b.

[0063] For example, for n=8, i.e. an eighth-wave plate in the referencearm, there will be a λ/4 or 90° phase difference between the spectraobtained from the reference and sample arms, which is sufficient forunambiguous reconstruction of the sample reflectivity from the complexspectrum thus obtained. For other values of n, i.e., n=4 (quarter-waveplate), n=2 (half-wave plate), other phase delays between the collectedspectra may also be obtained as needed for various phase-shiftinterferometry reconstruction algorithms. Although only 2 phase delaysmay be encoded into polarization, the polarization-based approach of theFDOCT embodiment 50 may be combined with the intrinsic interferometerport phase difference methods of the other FDOCT embodiments discussedabove to obtain at least 4 simultaneous spectra with different phasedelays.

[0064] As another example, the addition of a circulator into the sourcearm of the FDOCT embodiment 50, which may direct light into anotherpolarizing beamsplitter and two more spectrometers, would allow for thesimultaneous acquisition of spectra having 0°, 90°, 180°, and 270° phasedifferences. It will be clear to one of ordinary skill in the art thatnumerous other implementations of the inventive concept of polarizationencoding of phase may be used as extensions of the FDOCT embodiment 50,such as the rotation of all polarization-sensitive elements in theembodiment 50 by a fixed angle, or numerous alternative placements ofthe polarization-sensitive elements (including placing of the λ/n plate116 in the sample arm instead of the reference arm), while still fallingwithin the scope of the present invention.

[0065] Modes of operation of the FDOCT embodiment 50 of FIG. 5 includesacquiring simultaneous spectra having 0° and 180° phase delay by use ofa λ/4 waveplate in the reference arm. The spectra may be differenced inorder to eliminate sample and reference arm DC and sample armautocorrelation terms. The resulting difference spectrum may be inverseFourier transformed to acquire a one-sided A-scan. The length of thereference arm is preferably adjusted so that no reflections are observedfor z<0.

[0066] Another mode of operation for use with the FDOCT embodiment 50 ofFIG. 5 includes acquiring simultaneous spectra having 0° and 90° phasedelay between them and differencing them in order to eliminate sampleand reference arm DC and sample arm autocorrelation terms. These spectramay be taken as the real and imaginary parts, respectively, of thecomplex Fourier transform of the two-sided A-scan. The inverse Fouriertransform may then be performed on the complex data to obtain the A-scanfree of symmetry considerations.

[0067] Another mode of operation for use with the FDOCT embodiment 50 ofFIG. 5 includes acquiring simultaneous spectra having 0°, 90°, 180°, and270° phase difference between them, by use of a combination ofpolarization encoding of phase and intrinsic phase delay betweeninterferometer ports, as described above. Pairs of spectra having 180°phase delay between them may be differenced in order to eliminate sampleand reference arm DC and sample arm autocorrelation terms. Thedifferenced pairs of spectra may then be taken as the real and imaginaryparts, respectively, of the complex Fourier transform of the two-sidedA-scan. The inverse Fourier transform may be performed on the complexdata to obtain the A-scan free of symmetry considerations.

[0068]FIG. 6 illustrates a sixth FDOCT embodiment 60, in accordance withthe present invention. The FDOCT embodiment 60 includes a first fibercoupler 36 coupled to a first polarization controller 42 through a SMoptical fiber 41. The first polarization controller 42 is coupled to aSM optical fiber 51, which terminates in a reflector 46. The first fibercoupler 36 is also coupled to a second polarization controller 54through a SM optical fiber 52. The second polarization controller 54 isoptically coupled to a first lens 56 through a SM optical fiber 64. Thefirst lens 56 is optically coupled to a sample 62 through a scanningoptics 58 and second lens 61. The first fiber coupler 36 is also coupledto a third lens 126 through a coupler 64. The third lens 126 isoptically coupled to a polarizing beam splitter 120. The polarizing beamsplitter 120 is optically coupled to a lens 122 and a first detector 66.The polarizing beam splitter 120 is also optically coupled to a lens 124and a second detector 71.

[0069] It is preferable to have the capability for arbitrarysimultaneous dual phase delays between acquired spectra in a fiberinterferometer, since most practical OCT systems to date make use of theflexibility of fiber optic systems for medical and biologicalapplications. The FDOCT embodiment 60 illustrates one possibleimplementation of a fiber-optic interferometer for imaging, which usespolarization for phase encoding. In this embodiment, the light source 72is either polarized or a polarization element 112 (such as a fiberpolarizer) is used in the source arm. Preferably, the interferometer isconstructed from polarization-maintaining fiber (PMF), although previouswork in polarization-sensitive OCT has shown that non-PMF fiber is alsocapable of maintaining phase relationships between orthogonalpolarization states propagating through the fiber. In the FDOCTembodiment 60 of FIG. 6, a fiber polarization controller 42 in thereference arm may be used to simulate the λ/n waveplate 116 in the FDOCTembodiment 60, and the second polarization controller 54 in the samplearm may be used to correct for stress-induced birefringence in thesample arm fiber assembly.

[0070] Although the FDOCT embodiment 60 is just one example ofpolarization phase encoding in a fiber interferometer, any of the fiberinterferometers shown in the FDOCT embodiments described above could bealtered to use polarization phase encoding by the addition of a PBS andmatched pair of spectrometers to each output port 37 a and 37 b.Modifications of the FDOCT embodiments described above in this way wouldresult in the simultaneous collection of spectra with 4 phase delays(pairs of which are separated by 180°), since each of thoseimplementations have dual interferometer outputs.

[0071]FIG. 7 shows a generalized embodiment 100 of a FDOCT system, inaccordance with the present invention. The FDOCT 100 includes a signalmanipulator 12. The signal manipulator 12 is coupled to a referenceportion 14 by a coupler 20, and a sample portion 16 by a coupler 22. Thesignal manipulator 12 is also coupled to a detector 18 by a coupler 24.The signal manipulator 12 may also be coupled to a source 26 by acoupler 28. The sample portion 16 is coupled 31 to a sample 32.

[0072] Examples of signal manipulator 12 of FIG. 7 can include, but arenot limited to, the combination of the first optical circulator 34 andthe first fiber coupler 36 of FIG. 1, the second fiber coupler 76 andfirst fiber coupler 36 of FIG. 2, and the first fiber coupler 36 ofFIGS. 3 and 4. Additionally, the signal manipulator 12 of FIG. 7 mayalso correspond to the polarizer 110 and nonpolarizing beam splitter 114of FIG. 5, and the polarizing element 112 with the fiber coupler 36 ofFIG. 6.

[0073] The reference portion 14 of FIG. 7 may include, but is notlimited to, the combined first polarization controller 42, first phasemodulator 44, and reflector 46 of FIGS. 1 and 2. The reference portion14 of FIG. 7 may also include, but is not limited to, the second phasemodulator 84 and third polarization controller 88 of FIG. 3, and thefirst polarization controller 42 and first phase modulator 44 of FIG. 4.The reference portion 14 of FIG. 7 may also include, but is not limitedto, the combination of the λ/n plate 116 and reference mirror 118 ofFIG. 5, and the first polarization controller 42 and reflector 46 ofFIG. 6.

[0074] The sample portion 16 of FIG. 7 may include, but is not limitedto, the second polarization controller 54, first lens 56, scanningoptics 58 and second lens 61 of FIGS. 1, 2, 3 and 6. The sample portionof FIG. 7 may also include, but is not limited to, the combination ofthe second polarization controller 54, second optical circulator 102,first lens 56, scanning optics 58 and second lens 61 of FIG. 4, and thescanning optics 58 and second lens 61 of FIG. 5.

[0075] The detector 18 of FIG. 7 may include, but is not limited to, thefirst detector 66 and second detector 71 of FIGS. 1 and 2. The detector18 of FIG. 7 may also include, but is not limited to, the combination ofthe first and second detectors 66 and 71, and the third fiber coupler 94of FIG. 3, and the first and second detectors 66 and 71, and the fourthfiber coupler 98 of FIG. 4. Finally, the detector 18 of FIG. 7 mayinclude, but is not limited to, the first and second detectors 66 and71, the lens 124 and the lens 122, and the polarization beam splitter120 of FIGS. 5 and 6, as well as the third lens 126 of FIG. 6.

[0076] It should also be noted that the source 26 shown in FIG. 7 mayinclude, but is not limited to, the first source 72 of FIGS. 1, 2, 3, 5and 6. The source 26 of FIG. 7 may also include, but is not limited to,the first source 72 and second source 106 of FIG. 4.

[0077] Referring to FIG. 8, a detector 66 with imaging capabilitiessuitable for use with FDOCT embodiments 10, 20, 30, 40, 50, 60 and 100is shown. The detector 66 has dual input fibers 128 and 129 coupled toan input slit 136 of the detector 66. The detector 66 also includes anoutput area 130. The output area includes a first array 132 and a secondarray 134 which may receive two spectra, 138 and 140.

[0078] In operation, the detector 66 receives two multi-frequencysignals carried in the dual input fibers 128 and 129, at the input slit136. The input signals preferably have a phase difference between them,and the phase difference is preferably 90 degrees. Each input signal isthen dispersed according to frequency and the resulting spectra 138 and140, are directed onto the arrays 132 and 134. The arrays 132 and 134,may then measure power as a function of frequency for each spectrum 138and 140.

[0079] The previously described FDOCT embodiments 10, 20, 30, 40, 50, 60and 100 all preferably take advantage of the detection of multiplesimultaneous optical spectra in the detector arm of the interferometer,corresponding to multiple phase-delayed components of a complex FDOCTsignal. FIGS. 1-7 illustrate simple cases of dual-channel detection ofspectra separated by orthogonal (90°) or opposite (180°) phase obtainedthrough the use of intrinsic phase delays associate with interferometerports, or through polarization multiplexing. A combination of intrinsicand polarization-derived phase delays could also be used to obtain atleast four simultaneous phase delays, which would preferably be detectedin an equal number of spectral channels. Although four or moresimultaneous phase delays may be desirable to accommodate somephase-shift interferometry reconstruction algorithms, collection of twosimultaneous phases (optimally separated by 90°) would be one preferredembodiment of the invention, as that would allow for almost completeremoval of autocorrelation noise and calculation of complex double-sidedspectra with the least complexity and expense.

[0080] The light source 72 in the aforementioned embodiments ispreferably a low-coherence source. Multiple light sources 72 and 106,may also be used. The spectrometers 66 and 71 used in the FDOCTembodiments should preferably be selected for maximum optical throughputand optimal matching of their dispersion to the spectral content of thelow-coherence source, to avoid artifacts associated with the spatialfrequency response of the array (i.e., the dispersion should be chosenso that the spectrum nearly fills the detector array). Gratingspectrometers currently exhibit the optimal combination ofcharacteristics to satisfy these constraints, however other spectrometertypes may be used. If space utilization is not a serious constraint,prism-based spectrometers may give better throughput at the cost ofincreased required path length.

[0081] Preferably, the detector arrays utilized are photodiode arrayswith the maximum well depth available, and optimized for response in thewavelength range of the source. Using current detector array technology,this corresponds to the use of silicon photodiode arrays for the popular830 nm OCT window and for any other desired OCT spectral windows belowapproximately 1000 nm, and for InGaAs photodiode arrays for the popular1310 nm OCT window and for any other desired OCT spectral windows in thenear-infrared beyond 1000 nm. Charge-coupled device (CCD) arrays mayalso be used, however current-generation CCDs utilize silicon substratesand may thus be unsuitable for imaging at the popular OCT wavelengthsabove 1000 nm.

[0082] For simplicity, FIGS. 1-7 above illustrate the phase-delayedspectral channels as being dispersed and detected in separate detectors.As shown in FIG. 8, however, the spectrometers and arrays in thedetectors shown in each implementation can be replaced by a singleimaging spectrometer having a multiple-stripe or two-dimensionaldetector array, with the input fibers arranged in close verticalproximity to one another so as to have their spectra imaged onto theseparate rows (stripes) of the detector array. Such an arrangement wouldhave the significant advantages of allowing for optimal matching of thespectra placed on all channels (since all channels would use the samegrating and other spectrometer optics), as well as the cost and spacesavings achievable by using a single spectrometer. Dual-stripephotodiode arrays have been commercially available in the past, and alsothree-row CCD arrays designed for 3-color line scanning are currentlycommercially available. Two of the three rows of a 3-color line scannerarray could also be used in place of a dual-row array.

[0083] For a preferred embodiment of two FDOCT channels separated by 90°or 180°, a single dual-stripe photodiode array mounted onto an imagingspectrometer would be the preferred detector. Alternatively, atwo-dimensional CCD or photodiode array could be used to either a)simulate a dual-stripe array by using the binning capabilities of suchan array to collect dual simultaneous spectra, or b) collect more thantwo simultaneous spectra through an appropriate alternative binningalgorithm. However, two-dimensional CCDs still have significantwell-depth limitations, and large two-dimensional photodiode arrays arenot yet commercially available.

[0084] It should be noted that the term “optical circulator” is usedherein to mean any type of device capable of directional coupling ofelectromagnetic radiation incident on port 1 to port 2, whilesimultaneously coupling electromagnetic radiation incident on port 2 toport 3 Also, as used herein, a “fiber coupler” is used to mean anydevice which receives an input signal of electromagnetic radiation anddivides that signal between two output ports. It should be noted that asused herein, a fiber coupler may have multiple ports wherein each portcan serve as an input port for a selected pair of output ports as wellas function as an output port for a selected input port. The fibercoupler splitting ratio for all embodiments is ideally 50/50, howeverthis splitting ratio may be modified to account for nonideal performanceof other components, for example to compensate for the insertion loss ofcirculators or other elements. “Polarization controller” is used hereinto mean any semiconductor or bulk optical device used to selectivelymanipulate the polarization of an input signal and output themanipulated signal. “Optical fiber” is used to mean any device or set ofdevices used to direct electromagnetic radiation along a prescribedpath. Thus, “optical fiber” can mean a signal strand of opticallytransparent material bounded by a region of contrasting index ofrefraction, as well as mirrors or lenses used to direct electromagneticradiation along a prescribed path.

[0085] As used herein, “phase modulator” means any semiconductor or bulkdevice used to modulate or otherwise alter the phase of an inputelectromagnetic signal and output the manipulated electromagneticsignal. “Reflector” is used herein to mean any device capable ofreflecting an electromagnetic signal. Thus, “reflector” can be used tomean a mirror, an abrupt transmission in an index of refraction as wellas a periodically spaced array structure such as a Bragg reflector.“Scanning optics” means any system configured to sweep anelectromagnetic signal across a chosen area.

[0086] “Detector” is used herein to mean any device capable of measuringenergy in an electromagnetic signal as a function of wavelength.Additionally, “source” is used to mean any source of electromagneticradiation, and preferably means a low coherence source ofelectromagnetic radiation.

[0087] The foregoing embodiments and advantages are merely exemplary andare not to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art. In theclaims, means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures.

What is claimed is:
 1. A multi-frequency interferometric imager,comprising: a signal manipulator configured to receive a signal from asource and produce a reference signal and a sample signal; a referenceportion configured to receive the reference signal from the signalmanipulator and to send the reference signal back to the signalmanipulator; a sample portion configured to receive the sample signalfrom the signal manipulator and to send the sample signal back to thesignal manipulator, wherein the signal manipulator is further configuredto combine the reference signal and the sample signal and to produce afirst and a second measurement signal with a 180 degree phase differencetherebetween; and a detector portion configured to receive the first andsecond measurement signals.
 2. The multi-frequency interferometricimager of claim 1, wherein the reference portion comprises: a firstpolarization controller; a first phase modulator applicably coupled tothe first polarization controller; and a reference delay unit opticallycoupled to the first phase modulator.
 3. The multi-frequencyinterferometric imager of claim 1, wherein the reference portioncomprises a first phase modulator optically coupled to a firstpolarization controller.
 4. The multi-frequency interferometric imagerof claim 1, wherein the signal manipulator comprises a first fibercoupler optically coupled to a second fiber coupler, and wherein thereference portion is configured to receive a signal from the first fibercoupler and output a signal to the second fiber coupler.
 5. Themulti-frequency interferometric imager of claim 4, wherein the detectorportion is configured to receive a signal from the second fiber coupler.6. The multi-frequency imager of claim 6, wherein the detector portionis configured to receive a first signal and a second signal from thesecond fiber coupler, wherein the first signal and the second signalcomprise different phases.
 7. The multi-frequency interferometric imagerof claim 1, wherein the sample portion comprises a second polarizationcontroller optically coupled to scanning optics, and wherein the sampleportion is further configured to be optically coupled to a sample. 8.The multi-frequency interferometric imager of claim 1, wherein thesignal manipulator comprises an optical circulator optically coupled toa first fiber coupler.
 9. The multi-frequency interferometric imager ofclaim 8, wherein the reference portion and the sample portion are eachconfigured to receive a signal from the first fiber coupler.
 10. Themulti-frequency interferometric imager of claim 9, wherein the detectorportion is configured to receive a signal from the first fiber couplerand a signal from the second fiber coupler.
 11. The multi-frequencyinterferometric imager of claim 10, wherein the detector portioncomprises a first detector configured to receive a signal from the firstfiber coupler and a second detector configured to receive a signal fromthe optical circulator.
 12. The multi-frequency interferometric imagerof claim 1, wherein the signal manipulator comprises a first fibercoupler optically coupled to a second fiber coupler.
 13. Themulti-frequency interferometric imager of claim 12, wherein thereference portion and the sample portion are optically coupled to thefirst fiber coupler.
 14. The multi-frequency interferometric imager ofclaim 13, wherein the detector portion is configured to receive a signalfrom the first fiber coupler and the second fiber coupler.
 15. Themulti-frequency interferometric imager of claim 14, wherein the detectorportion comprises a first detector is configured to receive a signalfrom the first fiber coupler and a second detector configured to receivea signal from the second fiber detector.
 16. The multi-frequencyinterferometric imager of claim 1, wherein the signal manipulatorcomprises a first fiber coupler.
 17. The multi-frequency interferometricimager of claim 16, wherein the sample portion comprises: a firstpolarization controller; and an optical circulator optically coupled tothe first polarization controller.
 18. The multi-frequencyinterferometric imager of claim 17, wherein the detector portioncomprises a second fiber coupler, and first and second detectorsconfigured to receive signals from the second fiber coupler.
 19. Themulti-frequency interferometric imager of claim 18, wherein thereference portion and the sample portion are coupled to the second fibercoupler.
 20. The multi-frequency interferometric imager of claim 1,wherein the signal manipulator comprises: a polarizer; and anon-polarizing beam splitter optically coupled to the polarizer.
 21. Themulti-frequency interferometric imager of claim 20, wherein thereference portion comprises: a λ/n plate; and a reference delay unitoptically coupled to the λ/n plate.
 22. The multi-frequencyinterferometric imager of claim 21, wherein the sample portion comprisesscanning optics optically coupled to a sample.
 23. The multi-frequencyinterferometric imager of claim 22, wherein the detector portioncomprises a polarizing beam splitter optically coupled to a first andsecond lenses, a first detector configured to be optically coupled tothe first lens, and a second detector configured to be optically coupledto the second lens.
 24. The multi-frequency interferometric imager ofclaim 1, wherein the signal manipulator comprises a polarizing elementconfigured to be optically coupled to a fiber coupler.
 25. Themulti-frequency interferometric imager of claim 24, wherein thereference portion and the sample portion are optically coupled to thefiber coupler.
 26. The multi-frequency interferometric imager of claim25, wherein the detector portion comprises: first and second lensesoptically coupled to a polarizing beam splitter; a first detectorcoupled to the first lens; and a second detector coupled to the secondlens.
 27. The multi-frequency interferometric imager of claim 1, whereinthe reference arm is configured to selectively alter the polarization ofa signal.
 28. The multi-frequency interferometric imager of claim 1,wherein the reference arm is configured to selectively alter the phasesof a first and second polarization of a signal.
 29. The multi-frequencyinterferometric imager of claim 28, wherein a first polarization of asignal forms a first signal and a second polarization of a signal formsa second signal and, wherein the first signal and the second signal havean approximately π radian phase difference.
 30. The multi-frequencyinterferometric imager of claim 29 where the polarization of the firstsignal is orthogonal to the polarization of the second signal.
 31. Themulti-frequency interferometric imager of claim 1, wherein the sampleportion is configured to selectively alter the phase of a signal. 32.The multi-frequency interferometric imager of claim 1, wherein thedetector portion comprises a spectrometer and an array detector, andwherein the spectrometer and the array detector are configured tomeasure a power of a signal as a function of frequency
 33. Themulti-frequency interferometric imager of claim 30, wherein the arraydetector comprises a photo-detector array.
 34. The multi-frequencyinterferometric imager of claim 2, wherein the reference delay unitcomprises a reflector.
 35. The multi-frequency interferometric imager ofclaim 21, wherein the reference delay unit comprises a reflector.
 36. Adevice, comprising: a signal manipulator configured to receive a signalfrom a source; a reference portion configured to optically couple thesignal manipulator to a reference delay unit; a sample portionconfigured to optically couple the signal manipulator to a sample; and adetector portion configured to receive a signal from the signalmanipulator, wherein the signal can be used to measure a distance to thesample without scanning.
 37. The device of claim 36, wherein thereference delay unit comprises a reflector.
 38. A method, comprising:receiving a first spectra and a second spectra having a 180 degree phasedifference therebetween from a signal manipulator; determining adifference between the first and second spectra; and inverse Fouriertransforming the difference between the first and second spectra. 39.The method of claim 38, further comprising adjusting a portion of thefirst and second spectra to reduce reflections therein for z<0.
 40. Amethod, comprising: receiving a first spectra and a second spectrahaving a 180 degree phase difference therebetween from a signalmanipulator; determining a first difference spectra between the firstand second spectra; receiving a third and fourth spectra having a 180degree phase difference therebetween from a signal manipulator, whereinthe first and second spectra are orthogonal to the third and fourthspectra, respectively; determining a second difference spectra betweenthe third and fourth spectra; complex Fourier transforming the first andsecond difference spectra to produce a complex Fourier transform,wherein the first difference spectra comprises the real part of thecomplex Fourier transform and the second difference spectra comprisesthe imaginary part of the complex Fourier transform; and inverse Fouriertransforming the complex Fourier transform.
 41. The method of claim 40,wherein the first spectra has a 0 degree phase shift relative to areference spectra.
 42. The method of claim 41, wherein the phasedifference between the first spectra and the third spectra isapproximately 90 degrees.
 43. A method, comprising: creating a first anda second spectra; shifting the phase of the first and second spectrarelative to one another with a retarder until the first and secondspectra have a 180 degree phase difference; determining a differencespectra between the first and second spectra; and inverse Fouriertransforming the difference spectra.
 44. The method of claim 43, furthercomprising adjusting a portion of the first and second spectra to reducereflections therein for z<0.
 45. The method of claim 43, wherein theretarder is a λ/4 waveplate.
 46. The method of claim 43, wherein thefirst spectra has a 0 degree phase shift relative to a referencespectra.
 47. A method, comprising: receiving a first spectra and asecond spectra having a 0 degree phase difference therebetween from asignal manipulator; determining a first difference spectra between thefirst and second spectra; receiving a third and fourth spectra having a90 degree phase difference therebetween from a signal manipulator,wherein the first and second spectra are orthogonal to the third andfourth spectra, respectively; determining a second difference spectrabetween the third and fourth spectra; complex Fourier transforming thefirst and second difference spectra to produce a complex Fouriertransform, wherein the first difference spectra comprises the real partof the complex Fourier transform and the second difference spectracomprises the imaginary part of the complex Fourier transform; andinverse Fourier transforming the complex Fourier transform.
 48. Amethod, comprising: creating a first, second, third and fourth spectra,wherein there is a 90 degree phase shift between the first, second,third and fourth spectra, respectively, a 180 degree phase shift betweenthe first and third spectra, and a 180 degree phase shift the second andfourth spectra; determining a first difference spectra between the firstand third spectra; determining a second difference spectra between thesecond and fourth spectra; complex Fourier transforming the first andsecond difference spectra to produce a complex Fourier transform,wherein the first difference spectra comprises the real part of thecomplex Fourier transform and the second difference spectra comprisesthe imaginary part of the complex Fourier transform; and inverse Fouriertransforming the complex Fourier transform.
 49. The method of claim 48,wherein the first spectra comprises a first polarization, the secondspectra comprises a second polarization, the third spectra comprises athird polarization, and the fourth spectra comprises a fourthpolarization.